Bridging a stereo amplifier combines both channels in mono to roughly double the power output

Most professional stereo power amplifiers can be configured for bridged (mono) operation. In bridge mode, the two channels are driven with equal but inverted signals, and the load is connected between the positive output terminals of both channels rather than between positive and negative of one channel. The load sees twice the voltage swing of a single channel, so into the same impedance, power quadruples in theory (P = V²/R). In practice, and accounting for the minimum impedance the amplifier can drive, a bridged amplifier typically delivers roughly twice its rated stereo power into double the minimum impedance. For example, a 240 W/ch (8Ω) amplifier may deliver 800 W mono into 8Ω. Bridged mode is often used for powering subwoofers or long-throw arrays that require high power from a single amplifier.

Examples

A power amplifier rated at 240 W/channel into 8Ω stereo can deliver 800 W mono (bridged) into 8Ω. The 8Ω minimum in bridged mode corresponds to each channel ‘seeing’ 4Ω, which is its minimum stereo rating.

Assessment

Why does bridging approximately double (not quadruple) the practical output power? What impedance must the load be in bridged mode compared to stereo mode?